Impact of the thermomechanical load on subsurface phase transformations during cryogenic turning of metastable austenitic steels

Hendrik Hotz,Jan C Aurich,Benjamin Kirsch

doi:10.1007/s10845-020-01626-6

Hendrik Hotz, Jan C Aurich + Show 1 more

Open Access

https://doi.org/10.1007/s10845-020-01626-6

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Abstract

When machining metastable austenitic stainless steel with cryogenic cooling, a deformation-induced phase transformation from γ-austenite to α′-martensite can be realized in the workpiece subsurface. This leads to a higher microhardness and thus improved fatigue and wear resistance. A parametric and a non-parametric model were developed in order to investigate the correlation between the thermomechanical load in the workpiece subsurface and the resulting α′-martensite content. It was demonstrated that increasing passive forces and cutting forces promoted the deformation-induced phase transformation, while increasing temperatures had an inhibiting effect. The feed force had no significant influence on the α′-martensite content. With the proposed models it is now possible to estimate the α′-martensite content during cryogenic turning by means of in-situ measurement of process forces and temperatures.

Highlights

The surface integrity of a component significantly influences its performance in technical applications
The wide range of varied input parameters in cryogenic turning led to a great data range of process forces, temperatures and the resulting α′-martensite content
The high correlation coefficient r = 0.987 between the measured α′-martensite content ξm and the α′-martensite content ξc,np, calculated with the non-parametric model demonstrates on the one hand that the number of data sets and the quality of the measured data was sufficient and on the other hand that there is a high correlation between the thermomechanical load and the α′-martensite content formed in the workpiece subsurface during cryogenic turning

Summary

Introduction

The surface integrity of a component significantly influences its performance in technical applications. Brinksmeier et al (2018) described a general causal sequence of correlations starting from the input parameter of the machining process to the resulting thermomechanical loads during machining, the surface integrity after machining and the resulting functional properties. In addition to the formation of new grain boundaries, machining leads to a significant increase in the dislocation density below the surface. These alternations in the microstructure contribute to strain hardening and result in an increase in microhardness as demonstrated by Outeiro et al (2015) and Zhang et al (2018a). These alternations in the microstructure contribute to strain hardening and result in an increase in microhardness as demonstrated by Outeiro et al (2015) and Zhang et al (2018a). Jawahir et al (2016) concluded in a more recent keynote paper that the use of cryogenic cooling leads to a more pronounced grain refinement and stronger strain hardening compared with dry machining or machining with conventional flood cooling lubrication, due to the lower temperatures during machining

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